In the art of drilling wells to tap subterranean deposits of natural resources, such as gas, geothermal steam or oil, especially when drilling by the rotary method or the percussion method wherein cuttings must be removed from the bore hole, it is necessary to use a drilling fluid, as is well known to those skilled in the art. The subject is discussed more fully in U.S. Pat. No. 3,025,234. In addition to having the desirable rheological properties such as viscosity and gel strength, it is very important that drilling fluids exhibit a low rate of filtration or water loss, that is, the drilling fluid must prevent excessive amounts of fluid, or "filtrate", from flowing from the drilling fluid in the bore hole into the surrounding formation. The loss of water or other fluid from the drilling fluid is prevented by the formation of a filter cake which deposits from the drilling fluid and seals the wall of the bore hole.
Since most drilling fluids are "non-Newtonian" fluids, the apparent viscosity at given conditions of shear rate and shearing stress may change in a non-linear manner with certain parameters. This property makes it difficult to provide drilling fluids which will perform within acceptable ranges during the entire process of drilling a well.
Measurable variables which may characterize the apparent viscosity of a drilling fluid include the plastic viscosity, yield point and the rate and degree of gelation. The viscosity of a fluid normally decreases with an increase in temperature, but certain polymer additives or deflocculating agents may reduce, or even reverse, this tendency. The long-chain polymers which are most effective in achieving this effect are the most vulnerable to breakdown through oxidation, shear and thermal effects, i.e., the duration of exposure to high temperature drilling operations. Also, many such polymers tend to precipitate and/or lose viscosity as well as effectiveness as water loss additives when exposed to dissolved electrolytes, particularly when divalent metal cations such as Ca.sup.++ and Mg.sup.++ are present. In drilling fluids, the resulting vulnerability to breakdown is exacerbated by the density of the mud, which is directly related to the weighting agents required for a given formation pressure. The extremely high temperatures encountered in drilling geothermal wells also tend to cause early breakdown of mud additives.
Breakdown of water loss control polymers or deflocculants causes a large increase in the fluid loss accompanied by an increase in filter cake thickness. The thickness of filter cake will be determined by the solids content of the mud. High density drilling fluids with large amounts of weighting materials produce very thick filter cakes when fluid loss control is lost. These conditions often result in differential sticking of the drill string. It is desirable to develop additives which will enable drilling fluids to retain their proper viscosity and fluid content over a broader range of conditions.
Specifically, water base drilling fluids containing poly(N-vinyl-2-pyrrolidone/sodium 2-acrylamido-2-methylpropane sulfonate) provide excellent fluid loss control in hostile environments. Poly(N-vinyl-2-pyrrolidone/sodium 2-acrylamido-2-methylpropane sulfonate), however, provides sufficient viscosity in drilling muds that enough of this material cannot be incorporated into high solids muds to attain the degree of fluid loss control that is most desirable. It has now been found that the viscosity 2-acrylamido-2-methylpropane sulfonate) can be reduced by using a suitable additive in conjunction with the poly(N-vinyl-2-pyrrolidone/sodium 2-acrylamido-2-methylpropane sulfonate).
It is therefore an object of this invention to provide control of viscosity in drilling muds containing poly(N-vinyl-2-pyrrolidone/sodium 2-acrylamido-2-methylpropane sulfonate). It is another object of this invention to provide control of fluid loss in drilling muds containing poly(N-vinyl-2-pyrrolidone/sodium 2-acrylamido-2-methylpropane sulfonate). It is another object of this invention to provide drilling muds containing poly(N-vinyl-2-pyrrolidone/sodium 2-acrylamido-2-methylpropane sulfonate) in which the viscosity is controlled. It is still another object of this invention to provide drilling muds containing poly(N-vinyl-2-pyrrolidone/sodium 2-acrylamido-2-methylpropane sulfonate) in which the fluid loss from the drilling mud is controlled.
Other aspects, objects and alternative embodiments of this invention will become apparent upon reading this specification and the appended claims.